This article reviews the calculation methods of spiral duct friction loss in some Chinese and foreign literature, analyses the differences and reasons between the airflow in spiral duct and longitudinal seam duct. A formula for back-calculating the inner wall absolute roughness of spiral ducts using measured data is derived based on the Colebrook equation,and the inner wall absolute roughness of some diameters of spiral ducts is back-calculated by the friction factor calculation formula obtained by fitting with the measured data. The results show that the inner wall absolute roughness of spiral ducts is not a constant, but a function of the duct diameter and the axial average airflow velocity within the duct, and it can even be negative.